Nonvolatile semiconductor memory devices have been applied to portable phones, personal computers, household electrical appliances, gaming devices or the like, and widely used in the industrial world. The main nonvolatile semiconductor memory device currently being utilized in the industry is flash memory. In principle, flash memory is expected to face limit of miniaturization, and thus research on new nonvolatile semiconductor memory devices that will replace flash memory has been widely carried out. Among them, a study of resistance change memory that utilizes the phenomenon that application of voltage to a metal oxide film causes resistance to change (RRAM: Resistance controlled Random Access Memory) has been actively conducted recently, because the memory is more advantageous than flash memory in terms of limitation of miniaturization and because it is also capable of writing data at a high speed.
Although the study of the phenomenon that application of voltage to metal oxides such as nickel, iron, copper, titanium or the like changes resistance had been under way since 1960s (refer to Non-Patent Document 1), then, it was never put into practical use in actual devices. At the end of 1990s, it was proposed to apply to nonvolatile semiconductor memory device the fact that by giving voltage pulse for a short time to such oxides of manganese or copper having the Perovskite-type structure, deterioration of materials can be minimized and resistance can be increased or decreased. Then, it was demonstrated that a memory array of nonvolatile unit memory devices in which variable resistive elements using these metal oxides were combined with a transistor or a diode could be really formed on a semiconductor chip. This was reported in IEDM (International Electron Device Meeting) in 2002 (refer to Non-Patent Document 2), which triggered wide research to be undertaken in the semiconductor industry. Later, a similar approach was also taken in the research on oxides of nickel or copper carried out in 1960s, and memory devices produced by being combined with a transistor or diode were also reported.
All of these technologies are basically considered a same technology as they utilize resistance change in a metal oxide film to be induced by application of voltage pulse and use different resistance states as stored information in nonvolatile memory devices.
In addition, switching characteristics of the variable resistive element using the resistance change of the metal oxide film in accordance with the application of the voltage pulse include a bipolar type and a unipolar type. The switching characteristics of the both and practical applications thereof have already been reported in IEDM (refer to Non-Patent Document 2).
The bipolar switching implements switching between two resistance states by utilizing voltage pulses having two different polarities of plus and minus, having resistance of a variable resistive element transit from low resistance state to high resistance state with voltage pulse of any one of the polarities, and then having it transit from the high resistance state to the low resistance state with voltage pulse of the other polarity.
In contrast, the unipolar switching implements switching between two resistance states by utilizing voltage pulses having a same polarity and two different durations of long and short application (pulse width), having resistance of a variable resistive element transit from the low resistance state to the high resistance state with voltage pulse of one duration of application and then having it transit from the high resistance state to the low resistance state with voltage pulse of other duration of application.
Switching operations based on the two switching characteristics described above have benefits and problems, respectively. In fact, since the bipolar switching can implement transit time of several 10 ns or shorter as resistance increases or decreases, a memory device utilizing this can write accumulated data at a very high rate. However, due to use of application of voltage pulses of both positive and negative polarities, configuration of a circuit for implementing a semiconductor memory device becomes complex and chip size expands, thus leading to increased manufacturing cost.
On the other hand, as the unipolar switching can implement switching operation with voltage pulses of a single polarity, circuit configuration can be simplified, chip size can be smaller than that of the bipolar switching, and thus the former is better in terms of the manufacturing cost. In addition, as a combination of a diode and a variable resistive element can be used for a unit memory device, possible effect of current leakage from adjacent memory cells, which will be a problem when a memory cell array is configured as a cross point type, can be substantially reduced, thereby resulting in considerably improved electric characteristics in readout operation. However, as the unipolar switching uses two long and short voltage pulses, and, in particular, the former one needs the pulse width of a few μs, writing thereof takes 100 times longer than that of the bipolar switching. In addition, since the memory cell current during writing ranges from about several hundreds μA to a few mA as with the case of the bipolar switching, to write each memory cell, the unipolar switching also requires about 100 times as high power consumption as the bipolar switching. Thus, it is severely inferior to the bipolar switching in terms of performance during writing.
As a memory element whose resistance varies other than the RRAM, a phase change random access memory (PCRAM) has been developed. As the PCRAM uses a chalcogenide material as a resistor, a resistor material is crystallized or non-crystallized by heat generated when an electric pulse signal is applied, and the low resistance state or high resistance state is determined by a crystal state thereof. In general, although the resistance state of the PCRAM is changed by a unipolar switching operation similar to the RRAM, it takes as long as 100 ns to 1 μs for a set operation, as compared with a reset operation (change from the low resistance state to the high resistance state). According to the following non-patent document 3, the reset operation time is 30 ns while the set operation time is 120 ns.    Non-patent Document 1: H. Pagnia, et. al, “Bistable Switching in Electroformed Metal-Insulator-Metal Devices”, Physica Status Solidi (a), 108, pp. 11-65, 1988    Non-patent Document 2: W. W. Zhuang, et. al, “Novell Colossal Magnetoresistive Thin Film Nonvolatile Resistance Random Access Memory (RRAM)”, IEDM Technical Digest, pp. 193-196, December 2002    Non-patent Document 3: W. Y. Cho, et. al, “A 0.18 μm 3.0V 64 Mb Non-Volatile Phase-Transition Random-Access Memory (PRAM)”, 2004ISSCC Digest of Technical Papers, pp. 40-41, February 2004